Interpretive Summary: Gypsy moths are extremely common in much of the eastern United States. As such, their caterpillars are frequently encountered by people, many of whom respond allergically to the setae or hairs on the larvae. The research reported on here describes glands on the caterpillars that have been overlooked by most researchers, and reports the chemical identification of the secretion from these glands. The components of the secretion include known neurotransmitters, including gamma-aminobutyric acid, suggesting that the allergic properties of the hairs are not just due to mechanical irritation. All caterpillars in the gypsy moth family have similar glands. The compound responsible for the distinctive odor of the gypsy moth caterpillar secretion was also identified, and may be a cue used by parasites of the larvae to find these hosts.

Technical Abstract:
Gypsy moth caterpillars have unpaired dorsal abdominal glands on the sixth and the seventh segments, and pairs of smaller glands on the first to fourth abdominal segments. Normally, material from these glands becomes sticky and is regularly dispersed onto setae by the caterpillars, but if if larvae are held at saturated humidity for 3-4 days, droplets accumulate on the glands and remain fluid. The secretion is an aqueous mixture of low molecular weight molecules including the biogenic amine, gamma aminobutyric acid, short-chain hydroxy acids (e.g. alpha hydroxyisobutyric acid), and Krebs cycle acids (e.g. isocitric acid), plus higher molecular weight compounds (30,000 MW). 2-Isobutyl-3-methoxypyrazine, considered to be a warning odor in many aposematic insects, also occurs in the secretion in minute amounts (1 picogram/larva), yet is mainly responsible for the exudate odor which is detectable from individual caterpillars. Natural secretion from L. dispar larvae was a feeding deterrent to foraging fire ants in a laboratory bioassay. All Lymantriidae have dorsal abdominal glands; therefore, it is likely that secretion from these glands contributes to the irritating and allergenic properties associated with setae from tussock moth caterpillars